Method and Apparatus for Image and Video Coding Using Hierarchical Sample Adaptive Band Offset

ABSTRACT

A method and apparatus for image coding using hierarchical sample adaptive band offset. The method includes decoding a signal of a portion of an image, determining a band offset type and offset of a portion of the image, utilizing the band offset type and offset to determine a sub-band, and reconstructing a pixel value according to the determined offset value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 15/056,193,filed Feb. 29, 2016, which is a continuation of application Ser. No.13/671,722, filed Nov. 8, 2012, (now U.S. Pat. No. 9,277,194), whichclaims benefit of United States Provisional Application Nos. 61/557,036,filed Nov. 8, 2011, 61/593,578, filed Feb. 1, 2012, 61/595,777, filedFeb. 7, 2012, 61/618,264, filed Mar. 30, 2012, and 61/623,790, filedApr. 13, 2012, which are herein incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

Embodiments of the present invention generally relate to a method andapparatus for image and video coding using hierarchical sample adaptiveband offset.

Description of the Related Art

Sample adaptive offset (SAO) was introduced for the next generationvideo coding standard called high efficiency video coding (HEVC). FIG. 1is an embodiment depicting a block diagram for decoding Architecture ofhigh efficiency video coding with adaptive loop filtering (ALF) andsample adaptive offset. As shown in FIG. 1, SAO is applied afterdeblocking filtering process, usually, before adaptive loop filtering(ALF).

SAO involves adding an offset directly to the reconstructed pixel fromthe video decoder loop in FIG. 1. The offset value applied to each pixeldepends on the local characteristics surrounding that pixel. There aretwo kinds of offsets, namely band offset (BO) and edge offset (EO). BOclassifies pixels by intensity interval of the reconstructed pixel,while EO classifies pixels based on edge direction and structure. Incertain cases, the number of pixels increases but the number of bandsstays the same. As a result, the system becomes less efficient.

Therefore, there is a need for an improved method and/or apparatus for amore efficient image and video coding using hierarchical sample adaptiveband offset.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to a method and apparatusfor image coding using hierarchical sample adaptive band offset. Themethod includes decoding a signal of a portion of an image, determininga band offset type and offset of a portion of the image, utilizing theband offset type and offset to determine a sub-band, and reconstructinga pixel value according to the determined offset value.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is an embodiment depicting a block diagram for decodingarchitecture of high efficiency video coding with adaptive loopfiltering and sample adaptive offset;

FIG. 2 is a block diagram of a digital system;

FIG. 3 is a block diagram of a video encoder;

FIG. 4 is a block diagram of a video decoder;

FIG. 5 is an embodiment depicting band offset group classification;

FIG. 6 is an embodiment depicting edge offset pixel classificationpatterns; and

FIGS. 7A, 7B, and 7C depict examples of flag assignment.

FIG. 8 is an illustration of offsets.

FIG. 9 is an illustration of four bands.

FIG. 10 is an illustration of a band assignment order.

FIG. 11 is an illustration of a range of sub-bands.

FIG. 12 is an illustration of a range of bands.

DETAILED DESCRIPTION

FIG. 2 is a block diagram of a digital system. FIG. 2 shows a blockdiagram of a digital system that includes a source digital system 200that transmits encoded video sequences to a destination digital system202 via a communication channel 216. The source digital system 200includes a video capture component 204, a video encoder component 206,and a transmitter component 208. The video capture component 204 isconfigured to provide a video sequence to be encoded by the videoencoder component 206. The video capture component 204 may be, forexample, a video camera, a video archive, or a video feed from a videocontent provider. In some embodiments, the video capture component 204may generate computer graphics as the video sequence, or a combinationof live video, archived video, and/or computer-generated video.

The video encoder component 206 receives a video sequence from the videocapture component 204 and encodes it for transmission by the transmittercomponent 208. The video encoder component 206 receives the videosequence from the video capture component 204 as a sequence of pictures,divides the pictures into largest coding units (LCUs), and encodes thevideo data in the LCUs. An embodiment of the video encoder component 206is described in more detail herein in reference to FIG. 3.

The transmitter component 208 transmits the encoded video data to thedestination digital system 202 via the communication channel 216. Thecommunication channel 216 may be any communication medium, orcombination of communication media suitable for transmission of theencoded video sequence, such as, for example, wired or wirelesscommunication media, a local area network, or a wide area network.

The destination digital system 202 includes a receiver component 210, avideo decoder component 212 and a display component 214. The receivercomponent 210 receives the encoded video data from the source digitalsystem 200 via the communication channel 216 and provides the encodedvideo data to the video decoder component 212 for decoding. The videodecoder component 212 reverses the encoding process performed by thevideo encoder component 206 to reconstruct the LCUs of the videosequence.

The reconstructed video sequence is displayed on the display component214. The display component 214 may be any suitable display device suchas, for example, a plasma display, a liquid crystal display (LCD), alight emitting diode (LED) display, etc.

In some embodiments, the source digital system 200 may also include areceiver component and a video decoder component and/or the destinationdigital system 202 may include a transmitter component and a videoencoder component for transmission of video sequences both directionsfor video steaming, video broadcasting, and video telephony. Further,the video encoder component 206 and the video decoder component 212 mayperform encoding and decoding in accordance with one or more videocompression standards. The video encoder component 206 and the videodecoder component 212 may be implemented in any suitable combination ofsoftware, firmware, and hardware, such as, for example, one or moredigital signal processors (DSPs), microprocessors, discrete logic,application specific integrated circuits (ASICs), field-programmablegate arrays (FPGAs), etc.

FIG. 3 is a block diagram of a video encoder. FIG. 3 shows a blockdiagram of the LCU processing portion of an example video encoder. Acoding control component (not shown) sequences the various operations ofthe LCU processing, i.e., the coding control component runs the maincontrol loop for video encoding. The coding control component receives adigital video sequence and performs any processing on the input videosequence that is to be done at the picture level, such as determiningthe coding type (I, P, or B) of a picture based on the high level codingstructure, e.g., IPPP, IBBP, hierarchical-B, and dividing a picture intoLCUs for further processing. The coding control component also maydetermine the initial LCU CU structure for each CU and providesinformation regarding this initial LCU CU structure to the variouscomponents of the video encoder as needed. The coding control componentalso may determine the initial prediction unit and TU structure for eachCU and provides information regarding this initial structure to thevarious components of the video encoder as needed.

The LCU processing receives LCUs of the input video sequence from thecoding control component and encodes the LCUs under the control of thecoding control component to generate the compressed video stream. TheCUs in the CU structure of an LCU may be processed by the LCU processingin a depth-first Z-scan order. The LCUs 300 from the coding control unitare provided as one input of a motion estimation component 320, as oneinput of an intra-prediction component 324, and to a positive input of acombiner 302 (e.g., adder or subtractor or the like). Further, althoughnot specifically shown, the prediction mode of each picture as selectedby the coding control component is provided to a mode selector componentand the entropy encoder 334.

The storage component 318 provides reference data to the motionestimation component 320 and to the motion compensation component 322.The reference data may include one or more previously encoded anddecoded CUs, i.e., reconstructed CUs.

The motion estimation component 320 provides motion data information tothe motion compensation component 322 and the entropy encoder 334. Morespecifically, the motion estimation component 320 performs tests on CUsin an LCU based on multiple inter-prediction modes (e.g., skip mode,merge mode, and normal or direct inter-prediction) and transform blocksizes using reference picture data from storage 318 to choose the bestmotion vector(s)/prediction mode based on a rate distortion coding cost.To perform the tests, the motion estimation component 320 may begin withthe CU structure provided by the coding control component. The motionestimation component 320 may divide each CU indicated in the CUstructure into prediction units according to the unit sizes ofprediction modes and into transform units according to the transformblock sizes and calculate the coding costs for each prediction mode andtransform block size for each CU. The motion estimation component 320may also compute CU structure for the LCU and PU/TU partitioningstructure for a CU of the LCU by itself.

For coding efficiency, the motion estimation component 320 may alsodecide to alter the CU structure by further partitioning one or more ofthe CUs in the CU structure. That is, when choosing the best motionvectors/prediction modes, in addition to testing with the initial CUstructure, the motion estimation component 320 may also choose to dividethe larger CUs in the initial CU structure into smaller CUs (within thelimits of the recursive quadtree structure), and calculate coding costsat lower levels in the coding hierarchy. If the motion estimationcomponent 320 changes the initial CU structure, the modified CUstructure is communicated to other components that need the information.

The motion estimation component 320 provides the selected motion vector(MV) or vectors and the selected prediction mode for eachinter-predicted prediction unit of a CU to the motion compensationcomponent 322 and the selected motion vector (MV), reference pictureindex (indices), prediction direction (if any) to the entropy encoder334

The motion compensation component 322 provides motion compensatedinter-prediction information to the mode decision component 326 thatincludes motion compensated inter-predicted PUs, the selectedinter-prediction modes for the inter-predicted PUs, and correspondingtransform block sizes. The coding costs of the inter-predictedprediction units are also provided to the mode decision component 326.

The intra-prediction component 324 provides intra-prediction informationto the mode decision component 326 that includes intra-predictedprediction units and the corresponding intra-prediction modes. That is,the intra-prediction component 324 performs intra-prediction in whichtests based on multiple intra-prediction modes and transform unit sizesare performed on CUs in an LCU using previously encoded neighboringprediction units from the buffer 328 to choose the best intra-predictionmode for each prediction unit in the CU based on a coding cost.

To perform the tests, the intra-prediction component 324 may begin withthe CU structure provided by the coding control. The intra-predictioncomponent 324 may divide each CU indicated in the CU structure intoprediction units according to the unit sizes of the intra-predictionmodes and into transform units according to the transform block sizesand calculate the coding costs for each prediction mode and transformblock size for each PU. For coding efficiency, the intra-predictioncomponent 324 may also decide to alter the CU structure by furtherpartitioning one or more of the CUs in the CU structure. That is, whenchoosing the best prediction modes, in addition to testing with theinitial CU structure, the intra-prediction component 324 may also choseto divide the larger CUs in the initial CU structure into smaller CUs(within the limits of the recursive quadtree structure), and calculatecoding costs at lower levels in the coding hierarchy. If theintra-prediction component 324 changes the initial CU structure, themodified CU structure is communicated to other components that need theinformation. Further, the coding costs of the intra-predicted predictionunits and the associated transform block sizes are also provided to themode decision component 326.

The mode decision component 326 selects between the motion-compensatedinter-predicted prediction units from the motion compensation component322 and the intra-predicted prediction units from the intra-predictioncomponent 324 based on the coding costs of the prediction units and thepicture prediction mode provided by the mode selector component. Thedecision is made at CU level. Based on the decision as to whether a CUis to be intra- or inter-coded, the intra-predicted prediction units orinter-predicted prediction units are selected, accordingly.

The output of the mode decision component 326, i.e., the predicted PU,is provided to a negative input of the combiner 302 and to a delaycomponent 330. The associated transform block size is also provided tothe transform component 304. The output of the delay component 330 isprovided to another combiner (i.e., an adder) 338. The combiner 302subtracts the predicted prediction unit from the current prediction unitto provide a residual prediction unit to the transform component 304.The resulting residual prediction unit is a set of pixel differencevalues that quantify differences between pixel values of the originalprediction unit and the predicted PU. The residual blocks of all theprediction units of a CU form a residual CU block for the transformcomponent 304.

The transform component 304 performs block transforms on the residual CUto convert the residual pixel values to transform coefficients andprovides the transform coefficients to a quantize component 306. Thetransform component 304 receives the transform block sizes for theresidual CU and applies transforms of the specified sizes to the CU togenerate transform coefficients.

The quantize component 306 quantizes the transform coefficients based onquantization parameters (QPs) and quantization matrices provided by thecoding control component and the transform sizes. The quantize component306 may also determine the position of the last non-zero coefficient ina TU according to the scan pattern type for the TU and provide thecoordinates of this position to the entropy encoder 334 for inclusion inthe encoded bit stream. For example, the quantize component 306 may scanthe transform coefficients according to the scan pattern type to performthe quantization, and determine the position of the last non-zerocoefficient during the scanning/quantization.

The quantized transform coefficients are taken out of their scanordering by a scan component 308 and arranged sequentially for entropycoding. The scan component 308 scans the coefficients from the highestfrequency position to the lowest frequency position according to thescan pattern type for each TU. In essence, the scan component 308 scansbackward through the coefficients of the transform block to serializethe coefficients for entropy coding. As was previously mentioned, alarge region of a transform block in the higher frequencies is typicallyzero. The scan component 308 does not send such large regions of zerosin transform blocks for entropy coding. Rather, the scan component 308starts with the highest frequency position in the transform block andscans the coefficients backward in highest to lowest frequency orderuntil a coefficient with a non-zero value is located. Once the firstcoefficient with a non-zero value is located, that coefficient and allremaining coefficient values following the coefficient in the highest tolowest frequency scan order are serialized and passed to the entropyencoder 334. In some embodiments, the scan component 308 may beginscanning at the position of the last non-zero coefficient in the TU asdetermined by the quantize component 306, rather than at the highestfrequency position.

The ordered quantized transform coefficients for a CU provided via thescan component 308 along with header information for the CU are coded bythe entropy encoder 334, which provides a compressed bit stream to avideo buffer 336 for transmission or storage. The header information mayinclude the prediction mode used for the CU. The entropy encoder 334also encodes the CU and prediction unit structure of each LCU.

The LCU processing includes an embedded decoder. As any compliantdecoder is expected to reconstruct an image from a compressed bitstream, the embedded decoder provides the same utility to the videoencoder. Knowledge of the reconstructed input allows the video encoderto transmit the appropriate residual energy to compose subsequentpictures. To determine the reconstructed input, i.e., reference data,the ordered quantized transform coefficients for a CU provided via thescan component 308 are returned to their original post-transformarrangement by an inverse scan component 310, the output of which isprovided to a dequantize component 312, which outputs a reconstructedversion of the transform result from the transform component 304.

The dequantized transform coefficients are provided to the inversetransform component 314, which outputs estimated residual informationwhich represents a reconstructed version of a residual CU. The inversetransform component 314 receives the transform block size used togenerate the transform coefficients and applies inverse transform(s) ofthe specified size to the transform coefficients to reconstruct theresidual values.

The reconstructed residual CU is provided to the combiner 338. Thecombiner 338 adds the delayed selected CU to the reconstructed residualCU to generate an unfiltered reconstructed CU, which becomes part ofreconstructed picture information. The reconstructed picture informationis provided via a buffer 328 to the intra-prediction component 324 andto an in-loop filter component 316. The in-loop filter component 316applies various filters to the reconstructed picture information toimprove the reference picture used for encoding/decoding of subsequentpictures. The in-loop filter component 316 may, for example, adaptivelyapply low-pass filters to block boundaries according to the boundarystrength to alleviate blocking artifacts causes by the block-based videocoding. The filtered reference data is provided to storage component318.

FIG. 4 shows a block diagram of an example video decoder. The videodecoder operates to reverse the encoding operations, i.e., entropycoding, quantization, transformation, and prediction, performed by thevideo encoder of FIG. 3 to regenerate the pictures of the original videosequence. In view of the above description of a video encoder, one ofordinary skill in the art will understand the functionality ofcomponents of the video decoder without detailed explanation.

The entropy decoding component 400 receives an entropy encoded(compressed) video bit stream and reverses the entropy coding to recoverthe encoded PUs and header information such as the prediction modes andthe encoded CU and PU structures of the LCUs. If the decoded predictionmode is an inter-prediction mode, the entropy decoder 400 thenreconstructs the motion vector(s) as needed and provides the motionvector(s) to the motion compensation component 410.

The inverse scan and inverse quantization component 402 receives entropydecoded quantized transform coefficients from the entropy decodingcomponent 400, inverse scans the coefficients to return the coefficientsto their original post-transform arrangement, i.e., performs the inverseof the scan performed by the scan component 308 of the encoder toreconstruct quantized transform blocks, and de-quantizes the quantizedtransform coefficients. The forward scanning in the encoder is aconversion of the two dimensional (2D) quantized transform block to aone dimensional (1D) sequence; the inverse scanning performed here is aconversion of the 1D sequence to the two dimensional quantized transformblock using the same scanning pattern as that used in the encoder.

The inverse transform component 404 transforms the frequency domain datafrom the inverse scan and inverse quantization component 402 back to theresidual CU. That is, the inverse transform component 404 applies aninverse unit transform, i.e., the inverse of the unit transform used forencoding, to the de-quantized residual coefficients to produce theresidual CUs.

A residual CU supplies one input of the addition component 406. Theother input of the addition component 406 comes from the mode switch408. When an inter-prediction mode is signaled in the encoded videostream, the mode switch 408 selects predicted PUs from the motioncompensation component 410 and when an intra-prediction mode issignaled, the mode switch selects predicted PUs from theintra-prediction component 414.

The motion compensation component 410 receives reference data fromstorage 412 and applies the motion compensation computed by the encoderand transmitted in the encoded video bit stream to the reference data togenerate a predicted PU. That is, the motion compensation component 410uses the motion vector(s) from the entropy decoder 400 and the referencedata to generate a predicted PU.

The intra-prediction component 414 receives reference data frompreviously decoded PUs of a current picture from the picture storage 412and applies the intra-prediction computed by the encoder as signaled bythe intra-prediction mode transmitted in the encoded video bit stream tothe reference data to generate a predicted PU.

The addition component 406 generates a decoded CU by adding thepredicted PUs selected by the mode switch 408 and the residual CU. Theoutput of the addition component 406 supplies the input of the in-loopfilter component 416. The in-loop filter component 416 performs the samefiltering as the encoder. The output of the in-loop filter component 416is the decoded pictures of the video bit stream. Further, the output ofthe in-loop filter component 416 is stored in storage 412 to be used asreference data.

There are two kinds of offsets, namely band offset (BO) and edge offset(EO). BO classifies pixels by intensity interval of the reconstructedpixel, while EO classifies pixels based on edge direction and structure.For BO, the pixel is classified into one of two categories, the sideband or central band, as shown in FIG. 5. FIG. 5 is an embodimentdepicting band offset group classification.

FIG. 6 is an embodiment depicting edge offset pixel classificationpatterns. For edge offset, the pixels can be filtered in one of fourdirections, as shown in FIG. 6. For each direction, the pixel isclassified into one of five categories based on the value of c, where cis the intensity value of the current reconstructed pixel. The categorynumber can be calculated as sign(p0−p1)+sign (p0−p2), where p0 iscurrent pixel and p1 and p2 are neighboring pixels.

c<2 neighboring pixels;

c<1 neighbor && c==1 neighbor;

c>1 neighbor && c==1 neighbor;

c>2 neighbors; and

none of above (c=either neighbor).

On the encoder side, the SAO is LCU-based processing to estimate theoffsets for a LCU. Each LCU is allowed to switch between BO and EO.Thus, the encoder can signal the following to the decoder for each LCU:

sao_Plea_idx=type of SAO

sao_offset=sao offset value

Table 1 describes in more detail type of SAO or speciation ofNumSaoClass:

TABLE 1 sao_type_idx NumSaoCategory Edge type (informative) 0 0 Notapplied 1 4 1D 0-degree edge 2 4 1D 90-degree edge 3 4 1D 135-degreeedge 4 4 1D 45-degree edge 5 16 Central band 6 16 Side bandFor each LCU, a sao_type_idx is signaled followed by offset values foreach category. See FIG. 8.

In other embodiment, when SAO type is 2, edge offset class is signalledto indicate edge direction. When SAO type is 1, band position and offsetsigns are signalled, as shown in table 2 below:

TABLE 2 SAO type SaoTypeIdx[cIdx][rx][ry] (informative) 0 Not applied 1Band offset 2 Edge Offset

In one embodiment, the band offset scheme may have a reduced number ofoffsets. First, the entire level is divided into bands, for example, 4bands, as shown in FIG. 9. Each LCU can select one band by selectingappropriate band offset type, and selected band offset type is encodedinto bitstream. For the selected band offset type, the correspondinglevel range is again equally divided into 8 sub-band; hence, eachsub-band can have one offset. These offset values are encoded intobitstream.

At a decoder side band offset type and 8 offset values are decoded. Foreach pixel, it is determined in which sub-band it belongs to, andcorresponding offset value is added to the reconstructed pixel value. Inaddition, coding efficiency improvement can be achieved since number ofoffset to code is reduced by half.

In one embodiment, the coverage of each band is reduced compared to theconventional approach. To resolve this issue, the offsets of the firstand the last sub-band may include the pixels outside the band. Theoffset of the first sub-band covers pixels smaller than the minimumbound of the given band. The offset of the last sub-band covers pixelslarger than the maximum bound of the given band.

The number of band and sub-band maybe adaptively adjusted according tothe local or global characteristics of the given image of video. A BOtype number maybe also be assigned according to usage frequency.

Most frequently used BO type will be assigned the smallest number. FIG.5 shows an example, where the middle bands are assigned with 1 and 2,and the side bands are assigned with 3 and 4. FIG. 10 shows one sucharrangement.

In another embodiment, an increased number of band is used withoverlapped sub-band. The number of band maybe set to M, and number ofoffset can be set to N. M is used to signal the band where offset willbe applied. N is the number of sub-band for which the offset will beprovided.

For example, where M is set to 16 and N is set to 4, as shown below, theoverall pixel range is divided into 16 bands. The sub-band size is halfthe band in this case, and each band includes two sub-bands. Since thenumber of sub-band to signal offset is 4, the range of 4 sub-bands iswider than one band. Hence, there is overlap, as shown in FIG. 11. Insuch an example, 4 sub-bands from the second sub-band of 0th band to thefirst band of 2nd band are provided with offsets.

Whereas another example, where M is set to 32 and N is set to 4. Theoverall pixel level range is divided into 32 bands. In this case thesize of band is equal to that of sub-band. From the first sub-band inthe indicated band, N offsets are provided for each sub-band, where N is4, as shown in FIG. 12.

Note that the band indices maybe arranged in the order of their usagefrequency. In addition, in one embodiment, such a scheme maybe appliedfor each color component separately, or same offset can be applied forall the color components together. Thus, such a method and/or apparatusis capable of determining sign of offset according to category and notcoding it, which may remove visual artifact while facilitating animproved coding efficiency.

In one embodiment, for each band, a certain number of offset isprovided. One offset will cover one sub-band range. In other words, thepixels belonging to a sub-band will use corresponding offset. Forexample, four offsets can be provided for four sub-bands for theselected band. Since the range covered by these four sub-bands issmaller than the range covered by the band, the location in the band issignaled to specify the coverage of the offset. This can be signaledusing the sub-band index. For example, where there are four offsets, andif these offsets are provided from sub-band 3 in BO 0, four consecutivesub-bands including sub-band 3 can be covered, e.g., sub-band 3, 4, 5,and 6.

Since the pixel level range is hierarchically divided into band andsub-band, signaling of starting sub-band and offset values maybeefficiently performed in terms of coding efficiency. First, BO type issignaled, and then starting sub-band is signaled followed by the offsetvalues. It is possible to use fixed length coding or variable lengthcoding to code these parameters. Note that same scheme can be appliedfor both luma and chroma color components. It is possible to signal oneparameter set and apply it for all the color components. It is alsopossible to signal separate parameter set and independently apply it foreach color component.

It is also possible to adaptively change the band and/or sub-bandcoverage and/or number of offsets. For example, using the decoded pixelvalues after deblock filter process, the pixel level range within thegiven region can be determined by constructing a histogram, or by simplyfinding the minimum and maximum values.

In one embodiment, the band index and sub-band index are coded together.For example, where there are two bands and 16 sub-bands in each band,i.e., 1 bit flag can be used to indicate one of two bands, followed by 4bit flags to indicate one of 16 sub-bands. Such flags can be combined toform 5 bit flags.

Below is an example of simple way to map pixel values to these 5 bitflags, when there are two bands and 16 sub-bands in each band. First,pixel value is right shifted so that it belongs to the range of [0, 31].In case of 8 bit per pixel, it is right shifted by 3, which divide thewhole pixel range into 32 intervals. A 5 bit index is assigned to eachinterval from 00000 for the left most interval to 11111 for the rightmost interval.

FIGS. 7A, 7B, and 7C depict examples of flag assignment. The two mostsignificant bits are shown in FIG. 7A and are mapped in FIG. 7B. Thethree least significant bits are combined with the mapped two mostsignificant bits to generate new index. In this way, as shown in FIG.7C, the first bit of the five bit flag can indicate either middle bandor side band, and the other four bit can indicate 16 sub-band in eachband. Table 2 shows this mapping.

TABLE 2 before mapping after mapping 00 10 01 00 10 01 11 11

Let's take an example of this when pixel value is 123, which is 01111011in binary. After 3 bit right shift, this becomes 01111. Two mostsignificant bits, 01 is mapped to 00, and the three least significantbits, 111 is combined to this to form the final index, 00111. It can benoticed that this belongs to the middle band, since the first bit is 0.Hence, one advantage is a reduced buffer size to store SAO parameters byreducing number of band offset, which improves coding efficiency.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A method of a digital processor for image codingusing hierarchical sample adaptive band offset, comprising: determininga band offset type of four possible band offset types for at least aportion of the image, wherein each band offset type is divided into 8sub-bands; determining an offset related to the portion of the at leasta portion of the image, wherein each sub-band of the band offset typehas an offset; and encoding the determined band offset type and offsetfor the at least the portion of the image.
 2. The method of claim 1,wherein the encoding signals the band offset type and the sub-band forsignaling SAO type in 6 bits.
 3. A method of a digital processor forimage coding using hierarchical sample adaptive band offset, comprising:decoding a signal of a portion of an image; determining a band offsettype and offset of a portion of the image; utilizing the band offsettype and offset to determine a sub-band; and reconstructing a pixelvalue according to the determined offset value.
 4. The method of claim 1further comprising refraining from applying an offset value when areconstructed pixel is outside a predetermined range.
 5. A decoder forimage data decoding using hierarchical sample adaptive band offset,comprising: means for decoding a signal of a portion of an image; meansfor determining a band offset type and offset of a portion of the image;means for utilizing the band offset type and offset to determine asub-band; and means for reconstructing a pixel value according to thedetermined offset value.
 6. The decoder of claim 6 further comprisingmeans for refraining from applying an offset value when a reconstructedpixel is outside a predetermined range.